[U-Boot] [PATCH 1/2] drivers: net: driver for MDIO muxes controlled over I2C

[Bin Meng]

+Simon,

Hi Alex,

On Mon, Jul 15, 2019 at 7:45 PM Alexandru Marginean
<alexandru.marginean at nxp.com> wrote:
>
> On 7/13/2019 7:02 AM, Bin Meng wrote:
> > Hi Alex,
> >
> > On Fri, Jul 12, 2019 at 10:21 PM Alex Marginean
> > <alexandru.marginean at nxp.com> wrote:
> >>
> >> This driver is used for MDIO muxes driven over I2C.  This is currently
> >> used on Freescale LS1028A QDS board, on which the physical MDIO MUX is
> >> controlled by an on-board FPGA which in turn is configured through I2C.
> >>
> >> Signed-off-by: Alex Marginean <alexm.osslist at gmail.com>
> >> ---
> >>
> >> Depends on https://patchwork.ozlabs.org/project/uboot/list/?series=119124
> >>
> >>   drivers/net/Kconfig            |   8 +++
> >>   drivers/net/Makefile           |   1 +
> >>   drivers/net/mdio_mux_i2c_reg.c | 108 +++++++++++++++++++++++++++++++++
> >>   3 files changed, 117 insertions(+)
> >>   create mode 100644 drivers/net/mdio_mux_i2c_reg.c
> >>
> >> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> >> index 4d85fb1716..93535f7acb 100644
> >> --- a/drivers/net/Kconfig
> >> +++ b/drivers/net/Kconfig
> >> @@ -595,4 +595,12 @@ config FSL_ENETC
> >>            This driver supports the NXP ENETC Ethernet controller found on some
> >>            of the NXP SoCs.
> >>
> >> +config MDIO_MUX_I2C_REG
> >
> > I don't see current Linux upstream has any I2CREG support. I believe
> > you will upstream the Linux support too?
>
> Linux support is somewhat different so we won't have the same driver and
> binding there.  The main difference is that Linux has the concept of a
> mux controller, independent of the type of bus it muxes.  I didn't add
> this to U-Boot, not yet at least.
> This specific MDIO mux would be driven in Linux  by a pair of devices:
> - a mux controller with bindings defined by reg-mux.txt [1], this one
> implements the select function,
> - MDIO mux as a client of the controller, defined by
> mdio-mux-multiplexer.txt [2], which deals with the MDIO specific side of
> things.
>

Thanks for the detailed explanations! I thought people wanted to have
exact the same DT bindings as Linux so that we can sync-up upstream
kernel DT changes to U-Boot. That's why at first I checked whether
kernel has such bindings in place.

> In U-Boot I picked up the relevant properties from the two bindings, the
> MDIO mux device in U-Boot is supposed to implement select like a generic
> Linux MUX, while the uclass code deals with MDIO.
> I noticed U-Boot already has an I2C MUX class, along the lines of the
> MDIO one, I'm not sure if it's useful enough to introduce a new class of
> MUX controllers and then make all other kind of muxes depend on it.  For
> now at least the U-Boot mux drivers are simple enough and the extra
> class wouldn't help much.
>
> The other difference is that in Linux the mux controller is driven by
> the 'MMIO register bitfield-controlled multiplexer driver' [3], which is
> not I2C specific.  It is built on top of regmap which I also think is a
> bit too much for U-Boot.  For the U-Boot driver I just called I2C API
> directly and in that context it made sense to add I2C to the driver
> name, but there won't be an equivalent I2C based driver for Linux.
>

For your current implementation I believe that's OK, at least it is
verified on LS1028. But there must be a reason that Linux kernel chose
a DT that is different. I suspect the usage of kernel DT makes us stay
future-proof.

> I'd really appreciate some feedback on this.  I think the fairly simple
> approach in U-Boot is good enough, but I'm open to suggestions.

I am open on this topic too. Agreed that for U-Boot we always want a
simple implementation for device drivers. I am adding Simon for his
opinion on this topic as well.

>
>
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/mux/reg-mux.txt
> [2]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/net/mdio-mux-multiplexer.txt
> [3]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/mux/mmio.c
>

Regards,
Bin